Free CDL Air Brakes Questions and Answers — Questions and Answers
Question 1: To check the slack adjusters on S Cam brakes, which of the following procedures should you follow?
- Pull on the slack adjusters after draining the air from the system
- Use gloves and pull hard on each slack adjuster (Correct answer)
- Put the parking brake on and pull on the slack adjuster
- Insure that the slack adjusters move more than 1 inch where the push rod attaches to it
Correct answer: Use gloves and pull hard on each slack adjuster
To check the adjustment of S-cam brake slack adjusters, you should first ensure the parking brakes are off and the wheels are chocked. Then, using gloves for safety, pull hard on each slack adjuster. If the push rod moves more than about one inch (2.5 cm), the brakes are out of adjustment and need to be tightened.
Question 2: What is the normal air pressure in an air brake system that has been fully charged?
- 125 psi (Correct answer)
- 100 psi
- 150 psi
- 85 psi
Correct answer: 125 psi
A fully charged air brake system typically operates with an air pressure of approximately 125 psi. This is the 'cut-out' pressure where the compressor governor stops the compressor from pumping air into the reservoirs, maintaining optimal pressure for effective braking.
Question 3: How should one go about testing the air leakage rate correctly?
- Leave the engine running and release the parking brake
- Depress the brake pedal and monitor the air pressure gauge
- Turn off the engine and release the parking brake (Correct answer)
- Fully charge the air system and leave the engine running
Correct answer: Turn off the engine and release the parking brake
To correctly test the air leakage rate, the engine must be turned off to prevent the compressor from replenishing lost air. The parking brake should also be released (with wheels chocked) to allow for a more accurate assessment of leaks throughout the entire service brake system when the foot pedal is applied.
Question 4: What is brake stab?
- Using the parking brake to slow the vehicle in an emergency
- Applying the brakes as hard as possible without locking the wheels
- Applying a constant light pressure on the brakes
- Applying your brakes until the wheels lock up, releasing, and then reapplying when the wheels start rolling (Correct answer)
Correct answer: Applying your brakes until the wheels lock up, releasing, and then reapplying when the wheels start rolling
Brake stab (or stab braking) is a technique used in vehicles without ABS to stop quickly on slippery surfaces. It involves applying the brakes fully until the wheels lock, then releasing them until the wheels start rolling again, and immediately reapplying. This allows for maximum braking force while maintaining some steering control.
Question 5: Why do hydraulic brakes stop more quickly than air brakes?
- The air compressor adds a delay
- The driver has an increased reaction time
- It takes longer for the air to flow through the lines (Correct answer)
- All of the above
Correct answer: It takes longer for the air to flow through the lines
Hydraulic brakes generally stop more quickly than air brakes because hydraulic fluid is incompressible and transmits force almost instantly through the lines. Air, being compressible, takes a fraction of a second longer to travel through the lines and build pressure in the brake chambers, resulting in a slight delay compared to hydraulic systems.
Question 6: If your air compressor stops, what will happen?
- Your brakes will lock up
- You will have an immediate loss of your brakes
- Your brakes will not be affected
- You'll still have air pressure left in the air tanks to stop (Correct answer)
Correct answer: You'll still have air pressure left in the air tanks to stop
If the air compressor stops working, you will not immediately lose your brakes. The air tanks (reservoirs) store enough compressed air for several full brake applications. This stored air allows you to safely stop the vehicle, although you should pull over and address the issue as soon as possible.
Question 7: How should the "cut in" and "cut out" pressures on your air compressor governor be checked?
- Monitor the air pressure gauge
- Run the engine at a fast idle
- Step on and off the brake to reduce tank pressure
- All of the above (Correct answer)
Correct answer: All of the above
To check the 'cut-in' and 'cut-out' pressures of the air compressor governor, you need to monitor the air pressure gauge while running the engine at a fast idle. You then step on and off the brake pedal to gradually reduce tank pressure, observing when the compressor starts (cut-in) and stops (cut-out) pumping air. All these steps are necessary for a complete check.
Question 8: In a dual air system, how quickly should the air pressure build up?
- From 100 to 150 psi in 45 seconds
- From 70 to 125 psi in 60 seconds
- From 85 to 100 psi in 45 seconds (Correct answer)
- From 50 to 90 psi in 3 minutes
Correct answer: From 85 to 100 psi in 45 seconds
In a dual air system, the air pressure should build up from 85 psi to 100 psi within 45 seconds. This is a standard test to ensure the compressor is functioning efficiently and can adequately supply both air circuits, indicating a healthy air system. Failure to meet this standard suggests a problem with the compressor or system.
Question 9: What are the normal cut-in and cut-out pressures for the governor of an air compressor?
- Cut-in: 90 psi; Cut-out: 125 psi
- Cut-in: 100 psi; Cut-out: 150 psi
- Cut-in: 60 psi; Cut-out: 125 psi
- Cut-in: 100 psi; Cut-out: 125 psi (Correct answer)
Correct answer: Cut-in: 100 psi; Cut-out: 125 psi
The air compressor governor typically has a 'cut-in' pressure of around 100 psi, meaning the compressor starts pumping air when the system pressure drops to this level. It then has a 'cut-out' pressure of about 125 psi, at which point the compressor stops pumping to prevent over-pressurization of the air tanks.
Question 10: In the event that the air compressor leaks, what mechanism will maintain air in the tanks?
- The treadle valve
- The safety valve
- The one way check valve (Correct answer)
- The tractor protection valve
Correct answer: The one way check valve
A one-way check valve is installed between the air compressor and the air reservoirs. Its crucial function is to prevent compressed air from flowing back from the tanks into the compressor or out through a compressor leak. This ensures that the air tanks retain pressure even if the compressor fails or develops a leak.
Question 11: When the engine is off and the brakes are engaged, the air loss rate of a straight truck or bus shouldn't exceed what?
- 1 PSI in 30 seconds
- 3 PSI in 1 minute (Correct answer)
- 1 PSI in 1 minute
- 2 PSI in 45 seconds
Correct answer: 3 PSI in 1 minute
When performing an air loss test on a straight truck or bus with the engine off, parking brake released, and service brake applied, the air pressure should not drop more than 3 PSI in one minute. A greater loss indicates a leak that needs to be addressed for safety, as it could lead to brake failure.
Free CDL Air Brakes Questions and Answers
To check the slack adjusters on S Cam brakes, which of the following procedures should you follow?